In general, as shown in FIG. 6, a hydraulic circuit for a hydraulic shovel is arranged such that pressurized fluid flows discharged from two hydraulic pumps 150 and 152 are respectively entered to combined control valves 154 and 156, each of which comprises directional control valves 158, 160, 162, 164, 166 and 168, so as to respectively operate corresponding actuators such as an arm cylinder 170, a swivel motor 172, a left-traveling motor 174, a right-traveling motor 176, a boom cylinder 178 and a bucket cylinder 180.
In the hydraulic circuit mentioned above, the arm cylinder 170, the boom cylinder 178 and the bucket cylinder 180 respectively suffer from some drawbacks due to vacuum being often generated by a downward overload which is imposed on the tensive side thereof, resulting in occurrence of cavitation in the flow passages. In order to overcome such a disadvantage, to the respective directional control valves 158, 166 and 168 for the cylinders 170, 178 and 180 are added regeneration functions whereby a pressure loss on the supply-side is compensated by bypassing fluid on the return-side to the supply-side (the hydraulic circuit of this type is hereinafter referred to as a regeneration path). More preferably, exhausted fluid reducing function is also added to the directional control valves so as to control a drop speed of the actuator when it is subjected to the downward overload.
The directional control valve with the aforementioned regeneration path and its exhausted fluid reducing function are hereinafter described. For example, Japanese Patent Publications Nos. 46-15059 and 41-10446 disclose the directional control valve of this type.
Namely, FIG. 4(a) shows the directional control valve according to the Japanese Patent Publication No. 46-15059.
Referring to FIG. 4(a) a plunger 50 inserted into a plunger bore 52 of a housing is reciprocated from a neutral position to left and right positions in the bore so that a cylinder can operate to start and stop a vertical motion of an actuator cylinder on which a downward load is imposed.
Chambers 54, 56, 58, 60, 62, 64 and 66 are located in a sequentially spaced from the right to the left in the axial direction of the plunger 50. The chambers 56 and 64 are respectively connected to chambers 70a and 70b of a cylinder 70 while the chambers 54 and 66 are respectively connected to reservoirs 68.
Further, a pair of annular peripheral grooves 72 and 73 are provided at the center of the plunger 50 and isolated from each other by a plunger land 28. Plunger lands 79 and 80 are also formed on both end portions of the plunger.
Accordingly, when the plunger 50 is in the neutral position, the plunger lands 79 and 80 isolate, from the other chambers, the chambers 56 and 64 communicating with the chambers 70a and 70b of the cylinder 70, respectively.
An axial inner bore of the plunger 50 receives a pair of poppet valves comprising a check valve 112 and a control valve 114, a bypass valve 98 and a check valve 92 for preventing a load-drop . These valves have a restoring force due to elasticity of springs being used together with the valves.
As shown in FIG. 4(a), when the plunger 50 moves to the right position, fluid in a rod side chamber 70b of the cylinder enters into a bore 84 in front of a head of the bypass valve 98 arranged in the center portion of the axial inner bore of the plunger 50. Then, the bypass valve 98 is opened due to increase in pressure developed by the entering fluid so that a portion of the fluid flows into a supply passage 99.
The supply passage 99 is communicated through a fluid passage 88 formed on the plunger side wall, with a bore 82 formed on the head side of the check valve 92 which is arranged on the left side of the axial inner bore of the plunger 50. When the pressure force developed by the fluid entering from the supply passage 99 opens the check valve 92, the fluid is passed through a passage 90 formed on the side wall of the plunger 50 to communicate with the cylinder head side chamber 70a.
In operation, when the plunger 50 is in the illustrated position, fluid exhausted from a pump 128 is supplied to the head side chamber 70a of the cylinder 70 through the chamber 62, the fluid passage 88, the check valve 92, the passage 90 and the chamber 64. In this case, a portion of fluid in the cylinder rod side chamber 70b is supplied to the inside bore 84 of the bypass valve 98 through a fluid passage 106 and the rest is transmitted to a bore 104 through a passage 94.
Then, pressure force developed by the fluid is exerted on the head of a check valve 112 so as to open the check valve 112. Since a passage 108 formed on the housing side wall facing the plunger bore 52 is closed, the pressure force acts on a control valve 114 so as to move it to the open position. However, the check valve 112 and the control valve 114 can not be opened unless the pressure force exceeds a restoring force of a spring 122.
When the control valve 114 is opened, the check valve 112 in closing position is cracked to be opened. As a result, a return flow passing through the passage 94 into the bore 104 flows into the reservoir 68 through the check valve 112, the control valve 114, the passage 110 and the chamber 54.
Meanwhile, in the case an excess load acts on the cylinder so as to contract the rod side chamber 70b thereof, a piston 71 tends to be lowered at higher speed exceeding a capacity of the pump 128 by which the fluid is filled in a cavity of the cylinder head side chamber 70a, so that the feeding pressure of the pump is spontaneously decreased. In this case, a back pressure developed in a return path through which the return flow passes is exerted on the valve head 120 of the check valve 112. The back pressure is also exerted on the bypass valve 98 and serves to release the bypass valve 98 from a valve seat face so that a regeneration path can be opened.
To this end, a part of the return flow is transmitted to the supply passage 99 through the passage 94, the bore 84, the bypass valve 98, the passage 96, the annular peripheral groove 72 and the chamber 58. The fluid passing through such a flow path can compensate a short flow pumped from the pump 128 and be filled in the head side chamber 70a of the cylinder 70.
FIG. 4(b) shows the directional control valve according to the Japanese Patent Publication No. 41-10446.
Slidably inserted into a bore of the valve body 51 is a plunger 119 which can reciprocate from a neutral position to two opposite positions thereto. When the plunger 119 is moved to either one position, either one of central passages 67 and 69 is closed so that fluid pumped from a pump 65 is transmitted to either one of intake passages 74 and 76 through a supply passage 75 and a V-shaped connection passage 81.
The intake passages 74 and 76 are respectively connected to chambers 70a and 70b of the cylinder 70 through connection ports 55 and 57.
End portions of the V-shaped connection passage 81 are respectively communicated with a pair of peripheral grooves 83 and 91 formed outside the central passages 67 and 69 which are intersectionally communicated with a central portion of the bore in the valve body 51. The V-shaped connection passage 81 is communicated with the supply passage 75 through a back pressure check valve 85.
A pair of another peripheral grooves 86 and 87 are formed outside the peripheral grooves 83 and 91 in the longitudinal direction of the plunger 119 and communicate with the intake passages 74 and 76.
On the other hand, an exhaust passage 103 communicating with a reservoir 68 is formed in the central portion of the valve body 51 and communicated with a U-shaped return passage 89 extending through a relief valve 130 in two opposite directions. End portions 93 and 95 of the U-shaped return passage 89 respectively are formed on the positions outside and adjacent to the peripheral grooves 86 and 87 in the longitudinal direction of the plunger 119 and intersect the bore in which the plunger 119 is reciprocated.
In this case, the exhaust passage 103 and the U-shaped return passage 89 communicate with each other through the relief valve 130.
Further, the end portions 93 and 95 of the U-shaped return passage 89 are respectively communicated through the bore in the valve body with extensions 101 and 102 extending in parallel with the intake passages 74 and 76 which are respectively connected to the connection ports 55 and 57.
The extensions 101 and 102 are respectively communicated with the intake passage 74 and 76 through bypasses 97 and 100 and cavity control valves 105 and 107 having the same structure as the relief valve 130.
Accordingly, in the case fluid pressure in either of the intake passages 74 and 76 decreases less than that in the U-shaped return passage 89, for example, when cavitation tends to occur in the intake passage 74 or 76, pressure force developed by high pressure fluid in the U-shaped return passage 89 is exerted on the annular shoulder portion of either one of the cavity control poppet valves 105 and 107 communicating with the intake passages 74 and 76, respectively. Then, one of the poppet valves is separated from a valve seat so that return fluid is transmitted through the opened bypass 97 or 100 to the intake passage 74 or 76. To this end, the respective poppet valves are controlled such that a constant pressure difference can be maintained between the intake passages 74 and 76 and the U-shaped return passage 89. Under this operating condition, the poppet valve is separated further from the valve seat in case of reduction of the pressure in the intake passage 74 or 76. Thus, there is provided the directional control valve with a regeneration path.
FIGS. 5(a) and 5(b) show a directional control valve with exhausted fluid throttling function.
Referring to FIG. 5(a), in order to control gravitational drop speed when a load "W" is vertically imposed on a cylinder rod, the directional control valve 132 is provided with an exhausted fluid throttling device 133 with a check valve, which can reduce the fluid to be exhausted from the head side chamber 70a of the cylinder 70.
Referring to FIG. 5(b), a metering notch 135, a volume of which varies as the plunger moves in the axial direction, is formed on a plunger groove end 134. Exhausted fluid in the head side chamber 70a of the cylinder 70 can be throttled by contracting the volume of the metering notch in the vicinity of a piston stroke end when moving the plunger.
However, in recent years an operating pressure of the hydraulic equipment has been extremely increased. In the conventional directional control valve with the regeneration path shown in FIG. 4(a), the plunger is provided with a plurality of poppet valves. Therefore, a strength of the plunger may be reduced when efficiently forming a plurality of the passages in the plunger bore. Otherwise, even though the strength of the plunger is increased, the passages cannot be efficiently arranged and the manufacturing structure of the plunger may become complicated.
Although there has been proposed many other directional control valves having the plunger equipped with poppet valves therein, neither of the valves can overcome such problems as mentioned above.
In another type of a conventional directional control valve shown in FIG. 4(b), the above problems associated with the valve shown in FIG. 4(a) are eliminated. However, when the plunger is in the neutral position, the cavity control valve can not be operated due to the check valve arranged between the V-shaped connection passage and a main reservoir.
On the other hand, a conventional directional control valve with an exhausted fluid throttling function shown in FIG. 5(a), is provided in the outside position of the housing, with an exhausted fluid throttling device including a check valve. Some space is required to install the directional control valve. Further, since the check valve should be manufactured larger in order to form passages for a fluid flow, the exhausted fluid throttling device tends to be manufactured in larger size as a whole.
Referring to FIG. 5(b) another conventional directional control valve includes the plunger with the metering notch so as to throttle the exhausted fluid by changing the volume of the metering notch as moving the plunger. The directional control valve can effect space-saving and lower cost. However, a variety of the plunger should be employed according to gravitational drop speed of actuators to be used and should be manufactured on the basis of configuration of the metering notch.